Filename: 304-socks5-extending-hs-error-codes.txt
Title: Extending SOCKS5 Onion Service Error Codes
Author: David Goulet, George Kadianakis
Created: 22-May-2019
Status: Accepted

Note: We are extending SOCKS5 here but in terms, when Tor Browser supports
      HTTPCONNECT, we should not do that anymore.

0. Abstract

   We propose extending the SOCKS5 protocol to allow returning more meaningful
   response failure onion service codes back to the client.

   This is inspired by proposal 229 [PROP229] minus the new authentication
   method.

1. Introduction

   The motivation behind this proposal is because we need a synchronous way to
   return a reason on why the SOCKS5 connection failed.

   The alternative is to use a control port event but then the caller needs to
   match the SOCKS failure to the control event. And tor provides no guarantee
   that a control event will be emitted before the SOCKS failure or vice
   versa.

   With this proposal, the client can get the reason on why the onion service
   connection failed with the SOCKS5 returned error code.

2. Proposal

2.1. New SocksPort Flag

   In order to have backward compatibility with third party applications that
   do not support the new proposed SOCKS5 error code, we propose a new
   SocksPort flag that needs to be set in the tor configuration file in order
   for those code to be sent back.

   The new SocksPort flag is:

      "ExtendedErrors" -- Tor will report new SOCKS5 error code detailed below
                          in section 2.2 (once merged, they will end up in
                          socks-extension.txt).

   It is possible that more codes will be added in the future so an
   application using this flag should possibly expect unknown codes to be
   returned.

2.2. Onion Service Extended SOCKS5 Error Code

   We introduce the following additional SOCKS5 reply codes to be sent in the
   REP field of a SOCKS5 message iff the "ExtendedErrors" on the SocksPort is
   set (see section 2.1 above).

   The SOCKS5 specification [RFC1928] defines a range of code that are
   "unassigned" so we'll be using those on the far end of the range in order
   to inform the client of onion service failures:

   Where:

    * X'F0' Onion Service Descriptor Can Not be Found

      The requested onion service descriptor can't be found on the hashring
      and thus not reachable by the client.

    * X'F1' Onion Service Descriptor Is Invalid

      The requested onion service descriptor can't be parsed or signature
      validation failed.

    * X'F2' Onion Service Introduction Failed

      Client failed to introduce to the service meaning the descriptor was
      found but the service is not anymore at the introduction points. The
      service has likely changed its descriptor or is not running.

    * X'F3' Onion Service Rendezvous Failed

      Client failed to rendezvous with the service which means that the client
      is unable to finalize the connection.

    * X'F4' Onion Service Missing Client Authorization

      Tor was able to download the requested onion service descriptor but is
      unable to decrypt its content because it is missing client authorization
      information for it.

    * X'F5' Onion Service Wrong Client Authorization

      Tor was able to download the requested onion service descriptor but is
      unable to decrypt its content using the client authorization information
      it has. This means the client access were revoked.

3. Compatibility

   No new field or extension has been added. Only new code values from the
   unassigned range are being used. We expect these to not be a problem for
   backward compatibility.

   These codes are only sent back if the new proposed SocksPort flag,
   "ExtendedErrors", is set and making it easier for backward and foward
   compatibility.

References:

[PROP229] https://gitweb.torproject.org/torspec.git/tree/proposals/229-further-socks5-extensions.txt

[RFC1928] https://www.ietf.org/rfc/rfc1928.txt
